ABSTRACT ? Project 2: Defining and targeting the PAX3-FOXO1 interactome Alveolar rhabdomyosarcoma (ARMS) is a deadly childhood malignancy driven by the PAX3-FOXO1 fusion oncoprotein. While genetic approaches have rigorously validated this oncoprotein as a therapeutic target, the lack of compounds inhibiting PAX3-FOXO1 function has earned it the moniker of being ?undruggable?. Although ?undruggable? proteins can be pharmacologically targeted, doing so requires a thorough understanding of how the protein functions. It is well established that PAX3-FOXO1 exerts its transforming activities both through functional domains and physical interactions with other cellular proteins that either serve as modulators or co- regulators. Indeed, members of this team previously found that CHD4 co-regulates PAX3-FOXO1 function by indirectly associating with the oncoprotein via co-localization in chromatin neighborhoods. The overarching goal of this FusOnc2 Center is to advance the therapeutic tractability of the PAX3-FOXO1 fusion protein in ARMS by comprehensively identifying the druggable co-regulators, modulators, and intrinsic activities of PAX3-FOXO1. This Project?s objective is to systematically identify therapeutically exploitable components of the oncogenic PAX3-FOXO1 ?interactome? ? the catalog of proteins that interact with PAX3-FOXO1 that are also essential for its oncogenic activity. To identify the PAX3-FOXO1 domains and interacting proteins that mediate oncogenesis, this Project proposes a stepwise, comprehensive approach through the following Specific Aims: 1) Define the differential interactome of functional PAX3-FOXO1 using BirA proximity labeling; 2) Map PAX3-FOXO1 functional domains at amino acid resolution using saturation mutagenesis; 3) Define functional and combinatorial dependencies within the PAX3-FOXO1 interactome; 4) Credential interactome dependencies in cellular and animal models of PAX3-FOXO1 ARMS. The protein interactome defined in Aim 1 will be used to generate single- and dual-targeting CRISPR/Cas9 loss-of-function libraries in Aim 3. Work in Aim 2 will reveal the key functional domains and amino acid residues necessary for the oncogenic activity of the fusion oncoprotein, an achievement exploited throughout the Center to inform regions involved in PAX3-FOXO1 stability in Project 1 and prioritization of chemical probes in Project 3, and to refine the interacting proteins mediating PAX3-FOXO1 transformation in Aim 3 of this Project. Top candidates emerging from Aim 3 will then be nominated for in-depth analysis in Aim 4 using established cellular and animal models to define validated targets for near- and long-term development of drugs targeting either key interacting proteins themselves or their interactions with PAX3-FOXO1.